Unihemispheric slow-wave sleep

Unihemispheric slow-wave sleep (USWS) is sleep with one half of the brain while the other half remains alert. This is in contrast to normal sleep where both eyes are shut and both halves of the brain show reduced consciousness. In USWS, also known as asymmetric slow-wave sleep, one half of the brain is in deep sleep, a form of non-rapid eye movement sleep and the eye corresponding to this half is closed while the other eye remains open. When examined by low voltage electroencephalography (EEG), the characteristic slow-wave sleep tracings are seen from one side while the other side shows a characteristic tracing of wakefulness. The phenomenon has been observed in a number of terrestrial, aquatic and avian species.

Unique physiology, including differential release of the neurotransmitter acetylcholine, has been linked to the phenomenon. USWS offers a number of benefits, including the ability to rest in areas of high predation or during long migratory flights. The behavior remains an important research topic because USWS is possibly the first animal behavior which uses different regions of the brain to simultaneously control sleep and wakefulness. The greatest theoretical importance of USWS is its potential role in elucidating the function of sleep by challenging various current notions. Researchers have looked to animals exhibiting USWS to determine if sleep must be essential; otherwise species exhibiting USWS would have eliminated the behavior altogether through evolution.

Slow-wave sleep (SWS), also known as Stage 3, is characterized by lack of movement and difficulty of arousal. Slow-wave sleep occurring in both hemispheres is referred to as bihemisphereic slow-wave sleep (BSWS) and is common among most animals. Slow-wave sleep contrasts with rapid eye movement sleep (REM), which can only occur simultaneously in both hemispheres. In most animals, slow-wave sleep is characterized by high amplitude, low frequency EEG readings. This is also known as desynchronized state of the brain, or deep sleep.

In USWS, only one hemisphere exhibits the deep sleep EEG while the other hemisphere exhibits an EEG typical of wakefulness with a low amplitude and high frequency. There also exist instances in which hemispheres are in transitional stages of sleep, but they have not been the subject of study due to their ambiguous nature. USWS represents the first known behavior in which one part of the brain controls sleep while another part controls wakefulness.

...
Wikipedia